The present invention is directed to an improved photosensitive ceramic coating composition and more particularly a coating composition which functions as a precursor to a fired ceramic which has the ability to serve as a dielectric material particularly useful in formation of multilayer thick film circuits.
The present invention is directed to an improved photosensitive ceramic coating composition which can be fired in a substantially nonoxidizing atmosphere disclosed in Dueber et al. U.S. Pat. No. 4,613,560 issued Sept. 23, 1986. This patent discloses a coating composition which comprises an admixture of:
(a) finely divided particles of ceramic solids having a surface area-to-weight ratio of no more than 10 m.sup.2 /g and at least 75 wt. % of the particles having a size of 1-10 .mu.m. PA0 (b) finely divided particles of an inorganic binder having a surface area-to-weight ratio of no more than 10 m.sup.2 /g and at least 95 wt. %, of the particles having a size of 1-10 .mu.m, the weight ratio of (b) to (a) being 0.6-2, dispersed in an organic medium comprising PA0 (c) an organic polymeric binder selected from the group consisting of (1) homopolymer and copolymers of C.sub.1-10 alkyl acrylates, C.sub.1-10 alkyl methacrylates, alpha-methylstyrene and 0-2 wt. % ethylenically unsaturated carboxylic acid, amine or silane-containing compounds, (2) homopolymers and copolymers of C.sub.1-10 alkyl mono-olefins, (3) homopolymers and copolymers of C.sub.1-4 alkylene oxide and mixtures thereof, the binder comprising 5-25 wt. % basis total inorganic solids, and PA0 (d) a photoinitiation system, dissolved in PA0 (e) photohardenable monomer and PA0 (f) volatile nonaqueous organic solvent. PA0 (a) finely divided particles of ceramic solids having a surface area-to-weight ratio of no more than 10 m.sup.2 /g and at least 80 wt. % of the particles having a size of 1-10 .mu.m, and PA0 (b) finely divided particles of an inorganic binder having a glass transition temperature in the range from of 550.degree. to 825.degree. C., a surface area-to-weight ratio of no more than 10 m.sup.2 /g and at least 90 wt. % of the particles having a size of 1-10 .mu.m, the weight ratio of (b) to (a) being in a range from 0.6 to 2, dispersed in an organic medium comprising PA0 (c) an organic polymeric binder, and PA0 (d) a photoinitiation system, dissolved in PA0 (e) photohardenable monomer, and PA0 (f) an organic medium
As set forth in this patent processing involves the coating composition being (1) laminated to a ceramic substrate, (2) exposed imagewise to actinic radiation to effect hardening of the exposed areas of the film, (3) solvent developed to remove unexposed areas of the film, and (4) fired in a substantially nonoxidizing atmosphere to effect volatilization of the organic medium and sintering of the inorganic binder and ceramic solids.
As further background to the present invention multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on dielectric materials for this use. Heretofore, most of the dielectric materials used in multiple circuits have been conventional thick film dielectric compositions. These are comprised of finely divided particles of dielectric solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing, though they may be applied by other means as well.
Thick film materials of this type are very important and will continue to be so. However, when applying these thick film materials by screen printing, it is difficult to obtain fine line and space resolution. It is essential that all the screen printing variables such as screen quality, squeegee hardness, print speed, dispersion properties, etc., be most carefully controlled and constantly monitored to obtain good product yields. Similar problems exist, of course, with the use of thick film conductor and resistor materials.
One approach to this problem is (1) to apply a layer of the dielectric material to a substrate by means of dispersion in a photosensitive medium, (2) to expose the layer imagewise to actinic radiation, (3) to solvent develop the pattern to remove unexposed portions of the layer, and (4) to fire the remaining exposed portions of the pattern to remove all remaining organic materials and to sinter the inorganic materials.
A disadvantage of prior art compositions and particularly compositions disclosed in U.S. Pat. No. 4,613,560 is an organic solvent is necessary to develop such material after imagewise exposure to actinic radiation, i.e., an organic solvent removes areas of the composition which have not been exposed to actinic radiation without removal of areas which have been exposed.